The present invention relates to method of and an apparatus for machining a printed circuit board with high precision. More particularly, the present invention is directed to method of and apparatus for machining a printed circuit board suitable for forming closed holes and/or grooves with a high degree of dimensional precision in the direction of depth of such bores and grooves.
There has recently been an increasing demand for higher packaging density of wirings and electronic components on printed circuit boards, and multi-layered printed circuit boards which meet such demands are becoming popular. This also has given a rise to the demand for various mechanical processing or machining techniques to be effected on a printed circuit board, such as formation of closed holes for providing electrical connection between circuits of inner layers, face-up machining for forming recesses for situating integrated circuits (ICs) and other components, and so forth.
Such machining has to be effected with a high degree of precision in the depthwise direction, within a very small tolerance of .+-.0.05 mm or smaller. Japanese Patent Unexamined Publication No. 61-131804 discloses a machining method which permits a hole which reaches the desired circuit on an intermediate layer of a multi-layered printed circuit board to be formed, provided that such a circuit is exposed at an end surface of the multi-layered printed circuit board.
In general, however, a printed circuit board which constitutes one of the inner layers of a multi-layered printed circuit board usually has a predetermined circuit pattern which has been formed by etching, and is usually provided with a peripheral frame or cleat for forming reference holes therein. In such a case, it is not a practical method to control the drilling depth by means of an electrical signal exchanged between a drill and an electrode which is set on one end surface of the multi-layered printed circuit board.
Japanese Patent Publication No. 62-44337 shows a device in an industrial machine tool used for heavy industrial operations such as milling and drilling wherein the length of the tool in the tooling station is measured. However, such an arrangement is not acceptable for producing the dimensional precision needed in today's printed circuit boards, particularly relatively thin and closely packed surface mount technology (SMT) boards where blind holes must be drilled with great depth accuracy. The prior art device does not take into account dimensional variations in thickness of the work being processed which in the case of a printed circuit board can be 0.15 mm, the mounting condition of the tool which can vary in a circuit board drilling machine by .+-.0.02 mm, thermal distortion of the machine itself up to 0.02 mm, flatness of the machine table which can amount to 0.03 mm, and pitching and rolling of the machine table which can amount to 0.01 mm. While such variables might not be critical in heavier industrial machining operations of the type with which the machine of the aforesaid Publication No. 62-44337 is intended to be used, we have recognized them to be serious obstacles to the attainment of needed dimensional accuracy in drilling printed circuit boards.